Microcontrollers with integrated ADCs or stand-alone ADCs may comprise a plurality of analog input channels. These individual channels are generally connected to a multiplexer circuit which then couples a selected channel with the ADC proper. A channel is usually selected through a control or configuration register that provides for a plurality of selection bits, for example in a bit field, corresponding to the number of channels. A user can program such a register to select a respective channel by means of the multiplexor circuit or to automatically scan through a selected set of channels.
Crosstalk is a problem in any multiplexor circuit. For many fundamental reasons, if 1 of N inputs is selected and routed to a single output, all of the unselected inputs will still be present at the output to a measurable degree. In the case of an input multiplexor for an ADC converter, the baseline crosstalk is generally around −66 dB for an 80 MHz −1 dBFS input sinusoid, due to capacitive leakage of unselected inputs across the multiplexor (switch) element. This has not been improved by conventional circuit design techniques. Therefore, a need for an improved multiplexer input stage exists.